I wrote a plugin that provides an addToClasspath step for pipeline jobs:
https://github.com/cprice404/pipeline-classpath-step-plugin
It works well for cases where your Jenkinsfile doesn't need to import any classes, but rather just calls load on other groovy scripts, and those groovy scripts import classes. However, if I try to put any import statements directly in my Jenkinsfile, they seem to get processed before any other lines in the file, so the addToClasspath step doesn't get a chance to execute and the import fails.
In IRC it was mentioned that perhaps the pipeline jobs could automatically add .jenkins/src to their classpath, but it'd be much nicer if that was configurable somehow. e.g. if there was some property that could be set on the job to achieve it. If anyone can provide any hints on how that might be achievable I would be happy to try to work on the implementation further.
I poked around in the issue tracker and didn't see a ticket that appeared to capture this, but if I missed something please feel free to close it as a dupe.